(a) Field of the Invention
The present invention concerns the production of an oxygen enriched gas from air and by adsorption, according to a process of the "PSA" ("pressure swing adsorption") type carried out between an elevated pressure P.sub.M and a low pressure P.sub.m which is lower than the atmospheric pressure obtained by pumping.
(b) Description of Prior Art
The apparatuses of this type intended for the industrial production of oxygen by fractionning air over zeolites, for example of the 5A or 13X type, give oxygen enriched air up to 95% oxygen contents (the residual 5% essentially consisting of argon).
In a large number of applications, a production quality of 90/93% oxygen content is however sufficient. In this same range of contents, the quantities of oxygen required by the application may be from a few tons/day to a few hundreds of tons/day.
These known industrial apparatuses have been developed within the range of 10 to 50 T/d of oxygen, and they have appeared to be very cost competitive as compared to the oxygen obtained by cryogenic means and supplied in liquid form.
The different types of cycles proposed in these apparatuses generally comprise from 2 to 4 adsorbers, one of which is in production, while the other (or the others) are either being regenerated or are in intermediate phase (flushing, repressurizing . . . ).
Since these cycles have a duration which generally varies between 90 seconds and a few minutes, the volume of the adsorbers for a given cycle and a predetermined duration increases in proportion to the flow to be produced. The compliance with the rules of speed of flow of the gas in certain phases, to prevent the attrition of the absorbent, imposes a minimum cross section to the flow of gas, which, for large sizes, directly or indirectly becomes the limiting factor. In the case of adsorbers of vertical cylindrical shape and with vertical circulation of gas, the diameter of the adsorber becomes too large beyond a certain size of the apparatus (limitation of the diameter of the bottoms and sleeves, problems of transportation . . . ).
In the case of adsorbers of horizontal cylindrical shape and with vertical circulation of gas, which enable larger flows than int he preceding case to pass therethrough, with the same diameter, the use of large flows present problems of gas distribution in the internal collectors on both sides of the adsorbent, as well as of an increase of dead volume in these collectors. On the other hand, in same phases, kinetic phenomenons require minimum durations to prevent a degradation of the performances, which determines, for an adsorbent of given granulometry, an optimum duration of cycle.
Bearing in mind the requirements of attrition, kinetic and technological limitations, the limitation of such an apparatus for the production of oxygen is believed to be about 60 T/d.
In the case where an application requires more important quantities of over-oxygenated air, for example 300 T/d, the actual solution is either to install a plurality of units in parallel (for example 3 units of 50 T/d each, for the production of 150 T/d), or to go to the solution of utilizing cryogenic means.